Solving Techniques 5
X-Wing

When we have this situation, in either case
1
1
でも
1
1
a 1 can’t be in the cells marked X below.

An actual example can be seen below. There is a [1,2] pair in D2, D4.
Also, in row B, a 1 is only possible in B2 or B4 (due to the 1’s in the other boxes). This makes entering a 1 in rows B and D and columns 2 and 4 impossible.

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In case there are 1’s in 9 places.

It is the same as above, but with 9 cells. In the cells marked X below, a 1 can’t be entered.